What is the atmospheric pressure in a evacuated flask?
It depends upon whether you are measuring pressure inside or outside. Outside, it is the atmospheric pressure i.e. 980 to 1030 mbar (1013 mbar on an average) pressure. Average atmospheric pressure in different units is:
1 atm = 760 Torr=760 mm of mercury = 29.92 inches of mercury = 14.7 psi = 101.3 kN/m2 = 101.3 kPa=1.013 bar=1013 mbar
The pressure inside an evacuated container depends upon the strength of the container and the pump. Too much vacuum in a weak chamber and the chamber would collapse, as there would not be enough gas molecules inside to counterbalance the impact of millions of atmospheric gas molecules striking the outer surface.
The quality of a vacuum refers to how closely it approaches a perfect vacuum. Lower gas pressure means higher-quality vacuum. The perfect vacuum is a volume of space that is essentially empty of matter. In practice achieving complete emptiness is impossible. Even the outer space is not empty of matter. The quality indicators are low vacuum, medium vacuum, high vacuum, ultra high vacuum, extremely high vacuum and outer space. An ordinary roughing pump creates about 10^-5 torr pressure inside a container. Harder vacuums are attainable with more sophisticated pumps, temperature control and adsorbtion. Best attainable vacuums are now `10^-12` to `10^-15` torr, with the use of sophisticated pumps and high vacuum containers.